This concept is known as a self-directed IRA. I am going through the exact process right now rolling over a Roth to purchase shares in a privately held business. The major player in the field for the US is Equity Institutional - formerly Sterling Trust. The fee generally comes out to about 0.4% of AUM for smaller sums and goes down as asset value increases.
I contacted a company in CA named IRA Services Trust - lowest cost provider from what I could tell but I am still having trouble getting myself to trust the process. It seems like a fantastic opportunity for a major player to come in and dominate. Most custodians don't want the headaches. There are major tax compliance issues - annual valuations, prohibited transactions, IRS reporting, etc.
Last figures I saw were about 2% of IRA assets were in self-directed IRAs. I fully expect this to grow over time. The only well-known broker I could find was TD Ameritrade. They required 100,000 of liquid assets outside the IRA and then an annual fee of $250. Within TD Ameritrade, this IRA is referred to as holding a "non-standard asset" and an "alternative investment" - in case you are interested. I am sure you can google it or I have emails from them, if desired.
I would advise you to get the asset pre-approved by the provider's compliance office before rolling over any funds.